The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display using a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal.
Along with the recent developments of the so-called office automation (0A), 0A apparatuses, typically exemplified by word-processors and personal computers, have been widely used. As such 0A apparatuses have become prevalent in offices, there have been ever-increasing demands for portable-type 0A apparatuses that can be used in offices as well as outdoors, and there have been also demands for small-size and light-weight of such apparatuses. Here, liquid crystal displays have come to be widely used as one of the means to achieve such an objective. Liquid crystal displays not only achieve small-size and light-weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable 0A apparatuses that are driven by batteries.
The liquid crystal displays are mainly classified into the reflection-type and the transmission-type. In, the reflection-type liquid crystal displays, light rays that have been made incident on the front face of a liquid crystal panel are reflected by the back face of the liquid crystal panel so that an image is visualized by the reflected light. In the transmission-type liquid crystal displays, transmitted light from a light source (backlight) placed behind the back face of a liquid crystal panel is used to visualize an image. Although those of the reflection-type are inferior in visibility due to irregularity in the amount of reflected light that depends on environment conditions, they are inexpensive and widely used as display devices with mono-color (for example, black/white display, etc.) for such as calculators and watches. However, they are not suitable for display devices for personal computers, etc. which carry out a multi-color or full-color display. For this reason, in general, transmission-type liquid crystal displays are used as display devices for personal computers, etc. which carry out a multi-color or full-color display.
Here, currently-used color liquid crystal displays are generally classified into the STN (Super Twisted Nematic) type and the TFT-TN (Thin Film Transistor-Twisted Nematic) type based upon the liquid crystal type to be used. Although those of the STN type have comparatively low manufacturing costs, they are susceptible to cross-talk, and comparatively slow in response speeds; therefore, they are not suitable for display for animation pictures. In contrast, those of the TFT-TN type have higher display quality as compared with the STN type; however, since, at present, their liquid crystal panel has a light transmittance as low as 4%, a back-light with high luminance is required. For this reason, those of the TFT-TN type have greater power consumption due to the back-light, resulting in a problem in use of carrying battery power-source. Moreover, the TFT-TN type have other problems with the response speed, particularly slow in response speed for displaying half tones, narrow viewing angle, difficulty in adjusting the color balance, etc.
Moreover, in the conventional liquid crystal displays, a back-light of white light is used and the white light is selectively transmitted through color filters of three primary colors so as to perform a multi-color or full-color display; that is, those of the color-filter type have been generally used. However, in the color-filter type, a display pixel is constituted by a certain area including adjacent three color filters as one unit; therefore, the resolution is lowered to virtually one-third. Moreover, the application of the color filters reduces the transmittance of the liquid crystal panel, resulting in a reduction in the luminance as compared with the case without color filters.
In order to solve the above-mentioned problems, a liquid crystal display (Japanese Patent Application Laid-Open No. 7-281150, etc.) has been proposed in which a ferroelectric liquid crystal element or an anti-ferroelectric liquid crystal element, which has a high response speed to an applied electric field, is used as its liquid crystal element, and the same pixel is allowed to emit light rays with the three primary colors sequentially color by color so as to provide a color display.
FIGS. 1 and 2 are graphs respectively show the electro-optical characteristics of the ferroelectric liquid crystal and anti-ferroelectric liquid crystal. As shown in FIG. 1, the light transmittance of the ferroelectric liquid crystal varies depending on the polarity of an applied voltage. In the case of the plus application, the light-transmittance increases in response to the applied voltage, and in the case of the minus application, the light-transmittance becomes zero independent of the magnitude of the applied voltage. Moreover, as shown in FIG. 2, the light-transmittance of the anti-ferroelectric liquid crystal increases in response to the applied voltage in both of the cases of the plus and minus applications, and in the case of zero of the voltage application, the light-transmittance becomes zero. Therefore, in the case when these ferroelectric liquid crystal and anti-ferroelectric liquid crystal are applied to a liquid crystal display, a voltage corresponding to pixel data is supplied to each pixel of a liquid crystal panel and the light-transmittance is adjusted so that a display is available.
In a liquid crystal display using a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal having the above-mentioned electro-optical characteristics, a liquid crystal panel, which uses a ferroelectric liquid crystal element or an anti-ferroelectric liquid crystal element that is capable of a high-speed response in hundreds to several xcexc seconds order, and a back-light capable of emitting red, green and blue light rays in a time-divided manner are combined, and by synchronizing the switching of the liquid crystal element and the light emission of the back-light a color display is realized. In the case when the ferroelectric liquid crystal element or the anti-ferroelectric liquid crystal element is used as the liquid crystal material, the liquid crystal molecules are constantly maintained in parallel with the substrate (glass substrate) so that it is possible to provide a very wide viewing angle; therefore, no problem arises in practical use. Moreover, in the case when a back-light constituted by red, green and blue light-emitting diodes (LEDs) is used, it is possible to adjust the color balance by controlling currents flowing through the respective LEDs.
FIG. 3 is a block diagram that shows one example of the structure of a conventional liquid crystal display. To an image memory 52 included in a display control means 51 is supplied display data DD to be displayed on a liquid crystal panel 53 from, for example, an externally personal computer, etc. The image memory 52 temporarily stores the display data DD, and then transfers data of each pixel unit (hereinafter, referred to as pixel data PD) to a data driver 55, and the data driver 55 outputs the pixel data PD thus transferred. Moreover, the display control means 51 outputs a control signal to a scan driver 56, and the scan driver 56 controls the on/off operations of scan lines installed within the liquid crystal display panel 53. Furthermore, the display control means 51 supplies a driving voltage to a back-light 54 so as to allow an LED array included in the back-light 54 to emit light.
FIG. 4 is a time chart that shows one example of a conventional display control carried out in such a liquid crystal display. FIG. 4A shows timing of light emissions of the LEDs of respective red, green and blue colors of the back-light 54, FIG. 4B shows scanning timing of respective lines of the liquid crystal panel 53, and FIG. 4C shows color-emitting states of the liquid crystal panel 53.
As illustrated in FIG. 4A, the LEDs in the back-light 54 are allowed to emit light successively in the order of red, green and blue, for example, for each {fraction (1/180)} second, and in synchronism with this, the respective pixels of the liquid crystal panel 53 are switched on a line basis so as to provide a display. Here, in the case of a display of 60 frames for one second, one frame period is a {fraction (1/60)} second, and this one frame period is further divided into three sub-frames, each having a {fraction (1/180)} second. For instance, in the example shown in FIG. 4A, the red LED is allowed to emit light in the first sub-frame, the green LED is allowed to do so in the second sub-frame, and the blue LED is allowed to do so in the third sub-frame, respectively.
As illustrated in FIG. 4B, with respect to the liquid crystal panel 53, data scanning is carried out twice for each of the sub-frames of the respective red, green and blue colors. However, adjustments in timing are conducted so that the start timing (timing to the first line) of the first scanning (data-writing scanning) is coincident with the start timing of each sub-frame and so that the end timing (timing to the final line) of the second scanning (data-erasing scanning) is coincident with the end timing of each sub-frame.
During the data-writing scanning process, a voltage corresponding to pixel data PD is supplied to each pixel of the liquid crystal panel 53 so as to adjust the transmittance. Thus, it becomes possible to provide a full color display. In contrast, during the data-erasing scanning process, an electrical potential having the same voltage as that of the data-writing scanning process, but the reversed polarity thereto, is supplied to each pixel in the liquid crystal panel 53; thus, the display in each pixel in the liquid crystal panel 53 is erased so that it is possible to prevent a DC component from being applied to the liquid crystal.
However, in such a conventional liquid crystal display, there is a phenomenon in which, in the case of displaying an animation picture, the outline portion of the moving picture is observed as a rainbow colored portion. The following description will discuss the cause of this phenomenon.
FIG. 5 is an explanatory drawing that shows a model of an animation picture displayed on the liquid crystal panel by the above-mentioned conventional liquid crystal display. In FIG. 5, the axis of ordinate is the time axis, and the axis of abscissa represents a pixel on a certain line in the liquid crystal panel 53. Pixel numbers placed on the axis of abscissa are numbers that are assigned for convenience of explanation so as to identify pixels on the line shown in FIG. 5.
In this case, an animation picture displayed on the liquid crystal panel 53 is designed so that a white-color image corresponding to 8 pixels on the black background is allowed to shift six pixels for each frame in the increasing direction of the pixel numbers. Therefore, as illustrated in FIG. 5, in the red sub-frame (R-SF) within the nxe2x88x921 frame, red display data corresponding to pixel m to pixel m+7 are displayed. In the same manner, in the green sub-frame (G-SF) and the blue sub-frame (B-SF) within the nxe2x88x921 frame, green display data and blue display data corresponding to pixel m to pixel m+7 are respectively displayed.
Moreover, in the respective red, green and blue sub-frames within the n frame, red, green and blue display data, which correspond to pixel m+6 to pixel m+13 (not shown) that have shifted six pixels in the increasing direction of the pixel numbers as compared with those in the nxe2x88x921 frame, are respectively displayed. In the sub-frames within the succeeding frames, display data of the respective colors are displayed in the same manner.
When such an animation picture is observed, the observer views the image while shifting his or her view point following the shift of the image. Therefore, the observer has to shift his or her view point six pixels for each frame in the shifting direction of the image, as indicated by arrow A in FIG. 5.
The reason that the observer shifts the view point while viewing the animation picture is because the observer tries to always maintain the shifting image at the same position on his or her retina. Consequently, the observer recognizes an image as shown in FIG. 6.
FIG. 6 is an explanatory drawing that shows a model of an animation picture that the observer recognizes. In the same manner as FIG. 5, in FIG. 6, the axis of ordinate is the time axis, and the axis of abscissa indicates a pixel on a certain line in the liquid crystal panel. The xe2x80x9cresults of observationxe2x80x9d indicate an image that the observer actually recognizes, and they show that the closer the pitches of slanting lines, the darker the image recognized by the observer becomes. Here, arrow A corresponds to arrow A shown in FIG. 5, which indicates the shift of the observer""s view point.
Within the nxe2x88x921 frame, the red display data corresponding to pixel m to pixel m+7 is displayed in the red sub-frame, and from time t0 at which the red sub-frame starts and to time t1 at which the red sub-frame ends, since the view point is being shifted following the shift of the image, the red display data thus displayed is observed as if it were flowing in a direction opposite to the shifting direction of the view point (in a decreasing direction of the pixel numbers).
Then, from time t1 to time t2 at which the green sub-frame ends, since the view point has been further shifted, the green display data is observed as if it were further flowing in the decreasing direction of the pixel numbers as compared with the red display data. In the same manner, the blue display data is observed as if it were further flowing in the decreasing direction of the pixel numbers as compared with the green display data. As a result, as illustrated in FIG. 6, in the nxe2x88x921 frame, the display data of the respective colors are observed as if they were being drawn in the decreasing direction of the pixel numbers, with an increasing influence as the sub-frames further proceed.
Within sub-frames in the following frames, the display data of the respective colors are observed as if they were being drawn in the decreasing direction of the pixel numbers.
In the case when such an animation picture is observed, since the respective display data of red, green and blue are separated in the time direction, the image observed has degradation in the image quality on its outline portion, as illustrated in xe2x80x9cthe results of observationxe2x80x9d in FIG. 6. More specifically, even in the case of, for example, a white display, there are a portion having only a blue display, a portion having only blue and green displays, a portion having only a red display and a portion having only green and red displays, with the result that, in these portions, the image being observed is subjected to color aberration, and observed not as a desired white color, but as rainbow colors.
The present invention has been devised to solve the above-mentioned problems, and its objective is to provide a liquid crystal display which has an arrangement in which, after display data of the respective colors have been displayed in each frame, a back-light is turned off for a predetermined time so as to narrow an area having color separations, thereby making it possible to make less conspicuous the phenomenon causing rainbow colors at the outline portion of an animation picture to be observed.
Moreover, another objective of the present invention is to provide a liquid crystal display in which a back-light is controlled so as to differentiate the order of display-data colors to be displayed in respective sub-frames within three consecutive frames so that display data of the three colors, that is, red, green and blue, are always provided at the outline portion, thereby making it possible to avoid the outline portion of an animation picture from being observed as rainbow colors.
A liquid crystal display in accordance with a first invention, which is provided with a liquid crystal panel having a plurality of liquid crystal pixel electrodes arranged in a matrix form, a back-light, placed on the rear face of a liquid crystal panel, for emitting a plurality of different color light rays sequentially color by color, and a back-light control circuit which emits the back-light sequentially color by color, and turns the back-light off for a predetermined time cyclically.
A liquid crystal display in accordance with a second invention, which is provided with a liquid crystal panel having a plurality of liquid crystal pixel electrodes arranged in a matrix form and a plurality of switching elements placed correspondingly to the respective liquid crystal pixel electrodes, a back-light, placed on the rear face of the liquid crystal panel, having a plurality of light sources respectively emitting different color light rays, and a light-source driving control circuit for controlling the driving processes of the respective light sources of the back-light by allowing each light source of the back-light to emit light rays sequentially color by color in synchronism with display data for each light source emitting color in one frame, applied to the respective liquid crystal pixel electrodes, while driving the switching elements to turn ON/OFF corresponding to the display data, so as to carry out a color display, wherein the light-source driving control circuit controls the driving processes of the respective light sources so as to allow them to successively repeat a turn-on period for turning on light sources color by color in turn, and a turn-off period for turning all the light sources off.
The first invention is provided with the back-light, placed on the rear face of the liquid crystal panel, for emitting, for example, three-color light rays of red, green and blue sequentially color by color. After the back-light has emitted the three-color light rays respectively, the back-light control circuit turns the back-light off for a predetermined time. In this manner, the one frame is provided with a period of time in which the back-light is turned off.
The second invention is provided with the light-source driving control circuit for controlling the driving processes of the three-color light sources, for example, of the back-light. Thus, the light-source driving control circuit drives the second-color light source after having driven the first-color light source, and then drives the third-color light source. Further, after having driven the third-color light source, it turns all the light-sources off.
Color separations, which take place at the outline portion of an animation picture, are generated during a period of time in which the light rays of the respective colors, red, green and blue, are emitted. Therefore, the time during which the back-light is turned off is provided so that an area having color separations can be narrowed; consequently, it is possible to make less conspicuous the phenomenon in which the outline portion of an animation picture is observed as rainbow colors.
A liquid crystal display in accordance with a third invention, which is provided with a liquid crystal panel having a plurality of liquid crystal pixel electrodes arranged in a matrix form, a back-light, placed on the rear face of the liquid crystal panel, for emitting at least three color light rays, and a back-light control circuit which allows the back-light to emit at least three color light rays sequentially color by color so as to differentiate the order of the light emissions within at least three consecutive frames.
A liquid crystal display in accordance with a fourth invention, which is provided with a liquid crystal panel having a plurality of liquid crystal pixel electrodes arranged in a matrix form and a plurality of switching elements placed correspondingly to the respective liquid crystal pixel electrodes, a back-light, placed on the rear face of the liquid crystal panel, having light sources of three colors, and a light-source driving control circuit for controlling the driving processes of the respective light sources of the back-light by allowing the back-light to emit light rays sequentially color by color in synchronism with three color display data in one frame, applied to the respective liquid crystal pixel electrodes, while driving the switching elements to turn ON/OFF corresponding to the display data, so as to carry out a color display, wherein the light-source driving control circuit controls the driving processes of the respective light sources so that, in each frame within consecutive three frames, among a first light-emitting order including the first, second and third colors in this order, a second light-emitting order including the second, third and first colors in this order, and a third light-emitting order including the third, first and second colors in this order, the respective light sources are driven in such a manner that the light-emitting order of each frame is different from the light-emitting orders of the other two frames.
In accordance with the third invention, the back-light that emits three color light rays of red, green and blue sequentially color by color is placed on the rear face of the liquid crystal panel. Further, the back-light control circuit controls the back-light in such a manner that the order of the light emissions of the three color light rays from the back-light is different in each of three consecutive frames.
In accordance with the fourth invention, the light-source driving control circuit controls the driving processes of the three-color light sources of the back-light. Here, the light-source driving control circuit assigns any one of the first light-emitting order including the first, second and third colors in this order, the second light-emitting order including the second, third and first colors in this order and the third light-emitting order including the third, first and second colors in this order to each frame within three consecutive frames in a manner so as not to overlap with each other, and the respective light sources are driven in accordance with the order of light emissions.
In three consecutive frames, this arrangement makes it possible to avoid the orders of the light-emissions of the respective colors from coinciding with each other, with the result that the three-color display data of red, green and blue are always allowed to exist at the outline portion of an animation picture. Therefore, since no color separations take place, it is possible to prevent the outline portion of an animation picture from being observed as a rainbow colored portion.
Additionally, in this case also, since the observer views the animation picture while shifting his or her view point following the shift of the animation picture, the later the sub-frame in each frame, the greater the influence of the phenomenon in which the display data is observed as if it were drawn in a direction opposite to the shifting direction of the animation picture. Therefore, there is degradation (fuzziness).due to the difference in brightness in the animation picture. In other words, during a period of three consecutive frames, the three-color display data of red, green and blue are always allowed to exist in the outline portion of the animation picture; however, since there is a difference in the lengths of the time of existence (light emission time), a monochrome display consisting of bright white and dark white is observed in an area having the degradation in the image quality, for example, in a white display. However, as compared with the case in which rainbow colors are observed, this case is more advantageous since the degradation in the image quality is less conspicuous.
In a liquid crystal display in accordance with a fifth invention that relates to the liquid crystal display of the first invention, the back-light control circuit allows the plurality of different color light rays of the back-light to be emitted sequentially color by color so as not to have the same light-emitting order of colors in the consecutive plurality of frames.
In a liquid crystal display in accordance with the sixth invention, which is provided with a liquid crystal panel having a plurality of liquid crystal pixel electrodes arranged in a matrix form and a plurality of switching elements placed correspondingly to the respective liquid crystal pixel electrodes, a back-light, placed on the rear face of the liquid crystal panel, having light sources of three colors, and a light-source driving control circuit for controlling the driving processes of the respective light sources of the back-light by allowing the back-light to emit light rays sequentially color by color in synchronism with three color display data in one frame, applied to the respective liquid crystal pixel electrodes, while driving the switching elements to turn ON/OFF corresponding to the display data, so as to carry out a color display, wherein the light-source driving control circuit controls the driving processes of the respective light sources so that, in each frame within consecutive three frames, among a first light-emitting order including the first, second and third colors in this order, a second light-emitting order including the second, third and first colors in this order, and a third light-emitting order including the third, first and second colors in this order, the respective light sources are driven in such a manner that the light-emitting order of each frame is different from the light-emitting orders of the other two frames, and the driving processes of the respective light sources are controlled so that, in each of the frames, after the period for driving the third light source, a turn-off period for turning all the light sources off is provided.
In accordance with the fifth invention, the back-light for emitting the three-color light rays sequentially color by color is placed on the rear face of the liquid crystal panel. After the back-light has emitted the three-color light rays respectively, the back-light control circuit turns the back-light off for a predetermined time. In this manner, the period of time for turning the back-light off is provided in each frame. Moreover, the back-light control circuit controls the back-light so that the orders of the light-emissions of the respective colors are different from each other in three consecutive frames.
In accordance with the sixth invention, the light-source driving control circuit for controlling the driving processes of the three-color light sources of the back-light is provided. Here, the light-source driving control circuit assigns any one of the first light-emitting order including the first, second and third colors in this order, the second light-emitting order including the second, third and first colors in this order and the third light-emitting order including the third, first and second colors in this order to each frame within three consecutive frames in a manner so as not to overlap with each other, and the respective light sources are driven in accordance with the order of light emissions. Moreover, the light-source driving control circuit turns all the light sources off, after having driven the third light source, in each frame.
In three consecutive frames, this arrangement makes it possible to avoid the orders of the light-emissions of the respective colors from coinciding with each other, with the result that the three-color display data of red, green and blue are always allowed to exist at the outline portion of an animation picture. Therefore, since no color separations take place, it is possible to prevent the outline portion of an animation picture from being observed as a rainbow colored portion. Moreover, since the period of time for turning the back-light off is provided in each frame, it is possible to minimize the difference in the light-emission times of the three color light rays, and consequently to narrow the area having degradation in the image quality due to differences in brightness.
A liquid crystal display in accordance with a seventh invention that relates to the liquid crystal display of the second or sixth invention is characterized in that the turn-off period is set to be approximately a xc2xc frame time.
In accordance with the seventh invention, after the back-light has emitted the three-color light rays sequentially color by color, the period during which the back-light is turned off is set to be approximately xc2xc frame time. Here, the xc2xc frame time refers to a xc2xc of a period of time required for displaying one frame. In this case, the light-emitting time of the back-light is xc2xe frame time.
Color separations, which occur at the outline portion of an animation picture, are generated while the respective color-light rays of red, green and blue are being emitted. Since the turn-off time of the back-light corresponding to xc2xc frame time is provided in this manner, it is possible to narrow the area having color separations to xc2xe, and consequently to make less conspicuous degradation in the image quality occurring in the outline portion of the animation picture.
In a liquid crystal display in accordance with an eighth invention that relates to the liquid crystal display of the second or sixth invention, the turn-off period is set to be approximately xc2xd frame time.
In accordance with the eighth invention, after the back-light has emitted the three-color light rays sequentially color by color, the period during which the back-light is turned off is set to be approximately xc2xd frame time. Here, the xc2xd frame time refers to a xc2xd of a period of time required for displaying one frame. In this case, the light-emitting time of the back-light is xc2xd frame time.
As compared with the seventh invention, since the turn-off time of the back-light is made longer so that it is possible to further narrow the area having color separations, and consequently to make less conspicuous degradation in the image quality occurring at the outline portion of a moving image.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.